%% Imperative Programs

% Schema
%-------
database({assembly(Part:string, Subpart:string, Quantity:integer),
	part_cost(Basic_Part:string, Supplier:string, Cost:real, Time:integer),
	cost(Part:string, Cost:real)
	 }).

%Derivated predicates and rules
%------------------------------
% Update cost with all basic parts and their costs.
basic_costs <- part_cost(Basic_part, _, Cost, _),
	       +cost(Basic_part, Cost).
export basic_costs.

% Update cost with all assembled (non-basic) parts and their costs.
assembly_costs <- forever( assembly(Part, _, _), ~unresolved_part(Part),
			   tally_costs(Part, Total), +cost(Part, Total) ).
export assembly_costs.

% unresolved_part contains those parts for which
% we do not have as yet a cost in the cost relation.
unresolved_part(P) <- assembly(P, Sub, _), ~cost(Sub, _).
export unresolved_part(X).

% tally_costs sums up the costs of all sub parts making up a part.
tally_costs(P, Total) <- get_all(P, Set_of_subs),
			 aggregate(sum, Set_of_subs, Total).
export tally_costs(X,Y).

% get_all computes the sub cost of a sub part by multiplying its
% quantity times the unit sub part cost.
get_all(P, <Prod>) <- assembly(P, Sp, Qty),
		      cost(Sp, SpCost),
		      Prod = SpCost * Qty.
export get_all(X,Y).